1. Field of the Invention
The present invention relates to a camera, a storage control device for use in combination with a photoelectric conversion device, and a focus detecting device. More particularly, the present invention relates to a storage control device used in combination with a photoelectric conversion device, that is suitable for use in an auto focus detecting device of a camera or a video camera.
2. Description of the Related Art
A conventional focus adjustment apparatus has a sensor including a plurality of focus detection areas and a plurality of photoelectric conversion devices corresponding to the respective focus detection areas.
The storage control of the sensor is performed according to a method called AGC. In this method, the storage control is performed separately for each focus detection area by comparing AGC information corresponding to the amount of illumination light with a reference level.
The reference level is set so that the storage control is performed without saturation of an image signal output. The reference level can be selected from a plurality of values that have been defined previously so as to achieve an optimum combination of a reference level and an image signal amplification gain selected from a plurality of values that have been defined previously.
U.S. Pat. No. 5,218,395 discloses a focus detecting device including: first and second focus detection blocks; and monitor elements for monitoring the amount of incident light, the monitor elements being disposed adjacent to the respective focus detection blocks, wherein the charge storage times of the focus detection blocks are controlled according to the output of the corresponding monitor elements thereby detecting the focusing condition of a lens mounted on a camera.
In focus detecting devices of the type in which storage control is performed separately for two or more focus detection blocks as in the technique cited above, the reference level should be set so that when light coming from an object to be detected strikes equally all focus detection blocks, the image signals output by the line sensors are all equal to each other so as to provide good focus detection accuracy.
In practice, however, the image signal output varies owing to the following factors: performance variation of an imaging optical system used to project optical images of the focus detection areas onto the sensor; variation within a sensor in sensitivity or the ratio of AGC information to the image signal output; and performance variation from sensor to sensor. In general, the latter factor has the most significant influence.
FIG. 22 illustrates a common AGC operation. Referring to this figure, the variation in performance among focus detection blocks in a sensor will be described below. In FIG. 22, the horizontal axis represents storage time, and the vertical axis represents output voltages associated with the AGC information.
Voltages SA, SB, SC, and SD are reference levels that are defined depending on predetermined image signal gains. These reference levels are set so that when an AGC information signal reaches a reference level, if an image signal is amplified with a gain corresponding to that reference level, the amplified image signal is always equal regardless of which reference level is selected.
AGC information is read at time GADET. This AGC information is used to determine the gain to be employed in amplification of the image signal. Thus, an optimum image signal gain is selected on the basis of the AGC information read at time GADET, and a corresponding reference level is selected from a group consisting of SA, SB, SC, and SD.
In the example shown in FIG. 22, all three AGC information signals PB1, PB2, and PB3 are greater than SB. Thus, the gain corresponding to the reference level SA is selected in this case, and the output signal of the photoelectric conversion device is read when the AGC information signal reaches level SA thereby obtaining an image signal.
In FIG. 22, time CGSTP denotes an allowable maximum storage time. If the output signal of the photoelectric conversion device is very small and thus the AGC information signal does not yet reach any reference level at time CGSTP, then the storage operation is forced to be terminated.
In the example shown in FIG. 22, the focus detecting device includes three focus detection blocks wherein storage control is performed separately for each block, and each focus detection block is illuminated equally with light so that each focus detection block has equal brightness and contrast. Furthermore, in the example shown in FIG. 22, the characteristic variation of imaging optical systems is much smaller than variations in characteristic of the sensor. The three AGC information signals PB1 to PB3 correspond to the contrast obtained when the focus detection blocks are equally illuminated. However, there is a difference in gradient among these three AGC information signals PB1 to PB3. As a result, storage time, that is the time required for an AGC information signal to reach the selected reference level SA, has a variation among the focus detection blocks. That is, the storage time for the focus detection block having the AGC information signal PB1 is t0, that of PB2 is t1, and that of PB3 is t2.
The variation of the storage time results in a variation of the image signal output. Therefore, in a focus detecting device of the type in which the variation of the image signal output level causes a variation of focus detection accuracy among focus detection blocks, the focus detecting device has separate reference levels corresponding to respective focus detection blocks and each reference level is adjusted separately so that the variation among focus detection blocks and the variation among different focus detecting devices.
FIG. 23 illustrates AGC characteristics obtained by adjusting the AGC characteristics shown in FIG. 22 in the manner described above. The AGC information PB1 is employed as a reference signal, and the reference levels are adjusted separately for PB2 and PB3.
In the case of reference level SA shown in FIG. 22, the image signals output by the focus detection blocks corresponding to PB2 and PB3 may be equal to the image output signal obtained when the signal PB1 reaches SA1 level in FIG. 23 by employing a storage time which is the same as in PB1 or t1. Therefore, SA2 is employed as the reference level for PB2, and SA3 is employed for PB3. For SB1, SC1 and SD1, selection is performed in a similar manner.
In this way, all focus detection blocks for PB1 to PB3 have the same storage time t1, and the image signal output levels become equal to each other.
On the other hand, in focus detection devices of the type in which the above-described variation does not cause a variation of focus detection accuracy among focus detecting blocks, the focus detecting blocks have a common reference level, and this common reference level is adjusted so that the variation of the focus detection accuracy among different focus detecting devices is minimized.
To better meet user's requirements for a focus detecting device, the focus detecting device should have a greater number of focus detection areas than in the above-described example. However, in the focus detecting device having one reference level which is commonly used in all focus detection blocks, it is difficult to obtain a small enough variation among focus detection blocks when the number of focus detection blocks are great. This means that a large variation of focus detection accuracy occurs in a focus detecting device.
One known technique to reduce the above-described variation is to select a sensor so that the variation among focus detection blocks is minimized. However, severe requirements for the sensor cause a reduction in production yield of the sensor, and thus also causes an increase in production cost.
In the case where a focus detecting device has a plurality of reference levels corresponding to respective focus detection blocks and each reference level is adjusted separately, the complexity of a reference level circuit increases and thus greater power consumption is required whereas the variation among focus detection blocks and the variation among different focus detecting devices can be minimized.